For headlights, it is more and more common to use semiconducting light sources, especially light emitting diodes (“LED”s). Especially for adaptive headlights, a row or an array of LEDs is used to obtain an illumination pattern.
The emission of an LED is usually Lambertian. A semiconductor diode often comprises an active area, a sealant or side coating on all sides of the device and a corpus surrounding the actual light emitting medium. Due to the side coating, there is always a gap between the active areas of two adjacent LEDs, even if they touch each other. To gain efficiency and to close the gaps between the individual LEDs, it is common to use a collimation optic using total internal reflection (“TIR”). These collimators have individual “fingers” that are positioned over the individual LEDs, such that a “finger” is arranged over every LED. Usually the exit face of the collimator is imaged to the far-field (infinity) by a lens.
Concerning the gaps between individual LEDs, it is a problem that the collimators with individual entry faces for each LED are difficult to manufacture and difficult to adjust to all LEDs.
Concerning an LED-array, it is a problem that direct imaging systems with an LED array need a very large and complicated projection optic or they are not efficient due to the Lambertian emission characteristics of the LED.
WO2006096467A2 discloses a vehicle headlamp comprising a row of LEDs whose light is collected by a primary optical light guide having the form of a trapezoidal polypiped with an elongated rectangular entrance window facing the LEDs for light input. Such primary optical light guide outputs the collected light via an elongated rectangular exit window to a secondary light guide, e.g. a lens, which directs the light onto a road in front of the vehicle.
Therefore, it is an object of the invention to provide an improved LED lighting device that overcomes the problems described above.